Many applications require assemblies of diverse electronic circuit elements to accomplish a task. For example, antennas which transmit and receive radio frequency (RF) signals require the use of mixers, filters, amplifiers, capacitors, resistors and other circuit elements to generate and process RF signals. A variety of strategies have been used to optimize each individual type of device, or integrated circuit, and integrate them in ever smaller packages.
As frequencies surpassed 10 GHz, Monolithic Microwave Integrated Circuit (MMIC) circuits became increasingly important. These include gallium arsenide (GaAs) and indium phosphide (InP) Heterojunction Bipolar Transistor (HBT) and High-electron-mobility transistor (HEMT) MMICs, as well as microwave transmission lines, high-Q filters, and InP HBT ultra high-speed digital circuits, for example.
As MIMIC chips became smaller, a limiting factor in electronic assemblies was the package size and the space between packages on a printed wiring board (PWB) that formed the electronic assembly. To further decrease assembly size, designers turned to 3D interconnections between dies. One example of this is shown in FIG. 1A, in which a die 10 is stacked on another die 12, also referred to as an interposer, using solder 14 or other metallic bonds. The interposer 12 includes through-wafer vias 16 that connected die 10 to a substrate or packaging (not shown) using solder balls 18. In this prior art, the resistivity of the interposer 12, for example, silicon, is an impediment to operation of interconnect vias at microwave frequencies because this increases the loss to the RF signals. Another technique for decreasing package and PWB size is wafer-level packaging (WLP) wherein diverse types of Integrated Circuits (ICs) are effectively bonded together, forming electrical interconnects, before the wafer is diced into individual circuits. In other words, typical wafer fabrication processes are extended to include device interconnection or packaging steps.
A more complex example of the prior art is shown in FIG. 1B, in which IC dies 26 are mounted on a silicon interposer 28 having vias 30 similar to vias 16 of FIG. 1A. Interposer 28 is used to connect the smaller pitch solder balls 38 with the larger pitch solder balls 34, which are mounted to a packaging substrate 32, such as a printed wiring board (PWB) which has a larger feature size. Packaging substrate 32 may be mounted to further packaging (not shown) by solder balls 36.
Another factor driving development of electronic assemblies is the interest in integrating heterogeneous circuit functionality, for example a high-Q filter and a MIMIC. While it is known to combine various active elements such as transistors and diodes with passive elements such as resistors, capacitors and inductors on a wafer or chip, for example, this solution is not available when the function requires heterogeneous fabrication materials and different, possibly incompatible, manufacturing processes to produce them. As an example of this incompatibility, consider that typical MIMIC fabrication processes are done on wafers thinned to 2, 3, or 4 mil in order to reduce parasitic via inductance, but high-Q filters and ultra-low loss transmission lines, on the other hand, are typically fabricated on the thicker substrates and on substrates such as quartz which has a much lower dielectric constant than GaAs, GaN, or InP.
The prior art techniques as discussed above exhibit a number of problems when used with diverse circuit types and these problems become worse as frequencies climb above 20 GHz. For instance, a Printed Wiring Board (PWB), or softboard, has much larger via diameter, via spacing, and line/space rules than are possible with circuit elements fabricated in a microelectronics foundry, which causes radio frequency (RF) transitions between a MMIC and a PWB to typically have high loss and narrow bandwidth.
Thus, a need exists for a device and method for providing 3D integration of MMICs, filters and associated devices that includes improved high frequency performance, a significant size reduction, and better yield.